When a synthetic resin is to be colored with an MIC (Mold-In-Color) material, a pigment serving as a colorant is directly mixed and kneaded with the synthetic resin for example to thereby color the synthetic resin itself in a desired color. Particularly when a synthetic resin is to be colored in a metallic color, a paste-like aluminum pigment which is generally used for common paints is employed.
A paste-like aluminum pigment, however, contains an organic solvent which is typically mineral spirit or the like, and therefore is usually in a wet state. Due to this, if the paste-like aluminum pigment, as it is, is mixed and kneaded with a synthetic resin to color the synthetic resin, the organic solvent component remains in the synthetic resin and consequently, in some cases, the organic solvent component bleeds from the surface of a synthetic resin molded product into which the synthetic resin is molded. Further, since the aluminum pigment contained in the paste-like aluminum pigment is a metal in itself, the pigment's ability to contact (wettability) with the interface of the synthetic resin is insufficient, resulting in less satisfactory dispersiveness of the aluminum pigment in the synthetic resin. For these reasons, it has been difficult in some cases to sufficiently color a synthetic resin molded product in a metallic color.
In order to solve these problems, it has been proposed that particles of a metal such as aluminum are covered with a coating containing for example a terpene phenol resin and a low-density polyethylene resin to produce a free flowing granular material, the granular material is formed into pellets (so-called masterbatch), and the pellets are mixed and kneaded with a synthetic resin to color the resin in a metallic color (Japanese National Patent Publication No. 59-501550 (Patent Document 1)).
The proposed masterbatch is easy to handle, contains almost no organic solvent component like the one contained in the paste-like aluminum pigment, has adequate dispersiveness in the synthetic resin, and is therefore used for coloring a polyethylene film or the like in a metallic color.